Transition bandgap

Recent developments in solid state solutions of AlN/SiC/InN/GaN open up the possibility of a new generation of heterostructure devices based on SiC. Single crystal epitaxial layers of AlN/SiC/InN have been recently demonstrated by Dmitriev [4]. A whole range of solid state solutions has been grown. Recently Dmitriev et al [5] reported on an (AlNx-SiC,.x)-(AlNySiC,.y) p-n junction. Solid state solutions of AlN-SiC [6,7] are also expected to lead to direct gap ternary materials for UV and deep blue optoelectronics, including the development of visible lasers. The direct to indirect bandgap transition is predicted to occur at between 70 and 80 % of AIN in SiC. 

Fluorescence Ufetimes were very similar for both reduction approaches. In both cases, fluorescence decays (Figure 6.5) required a three-exponential fit with average lifetimes of 6 ns when reducing with IiAlH4 and 4.7 ns when with NaBH4. These short lifetimes reveal a rapid recombination associated with direct bandgap transitions in silicon nanocrystals. 

Due to many structural similarities to the solid-state structures, the soluble polynuclear complexes [Zn4(SPh)io] (17a), [Cd4(SPh)io] (17b), and [CdioS4(SPh)i6] (18) (Figure 12), may be considered as molecular models for the photophysical properties of the semiconductors ZnS and CdS. Lowest energy bands in the absorption spectra have been assigned to LMCT transitions. These can be viewed as the molecular equivalents of the bandgap transition between the... 

Optical transitions in semiconductors can also involve locahzed states in the band gap. These become particularly important for semiconductors in nanocrystalline form (seefollowing). Sub-bandgap transitions can be probed with photons of energy below the threshold defined by Eg. 

S-band hydrogen-terminated surface From near-infiared to blue 400-1,300 A few ns to 150 ps Yes Quantum confinement in Si nanocrystals indirect bandgap transitions blueshift upon size reduction (Cullis et al. 1997 Bisi et al. 2000 Wolkin et al. 1999)... 

Hot PL band From yellow to blue 425-630 ps range 0.01 % Quantum confinement in Si nanocrystals direct bandgap transitions observed only under high excitation redshifi upon size reduetion (de Boer etal. 2010)... 

Recent developments such as fast phonon-less transitions from carbon-terminated nanocrystals (Dohnalova et al. 2013), fast direct bandgap transitions (Prokofiev et al. 2009 de Boer et al. 2010), and very high values of luminescence quantum efficiencies of silicon nanocrystals in layers (in porous silicon, 23 % (Gelloz et al. 2005 Gelloz and Koshida 2005), and in other assemblies, 18-100 % (Ledoux et al. 2000) and 60 % (Jurbergs et al. 2006)) show that the luminescence of nanocrystalline silicon is progressively paving its way toward applications. 

Transitions between the valence and conduction bands in semiconductors. These are the origin of colour in semiconductor pigments and quantum dots. In addition to colour from bandgap transitions, introduction of dopants or impurities into semiconductors leads to localised energy levels on the dopant or impurity atoms/molecules (so-called colour centres) and transitions between these levels and those of the semiconductor conduction and valence bands become possible the colour in blue and yellow diamonds arises firom these types of transitions. 

Schematic of direct and indirect bandgap transition. The vertical transitions are allowed direct transitions. The indirect transition from the fc = 0 to the minimum of the conduction band requires the photon to combine with a phonon in order to conserve momentum.

Consequently the absorption at 3.6 eV determined in this study can be ascribed to a bandgap transition for the conducting form of polypyrrole. Bredas and Street(54) suggested that absorptions at 0.7, 1.4 and 2.1 eV present for polypyrrole doped at low levels, were characteristic of polaron species. Furthermore, as the doping levels increased, features at 1.0 and 2.7 eV developed which were taken to be indicative of bipolaron formation. Batz et al,(35) used HREELS to study doped polypyrrole films, and they noted that a low doping levels polarons caused loss features to be established at 2.0 and 2.5 eV. This structure was attributed to the fact that isolated polarons interacted with each other to produce a band with an appreciable width. 

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